The objective of this work is to understand how water is oxidized to from oxygen in photosynthetic organisms. The health of all aerobic organisms, including man, is critically dependent upon this process because it is the only significant global source of atmospheric oxygen in the biosphere. We seek to identify the structures of the chemical intermediates that form during this four step, enzyme catalyzed reaction. The structures of and the roles played by four manganese ions which function at the active site are the key objectives of this research. We introduce two previously unexplored physical methods for studying the water oxidizing site in photosynthetic membranes. There are three proposed topics of research: 1) time resolved magnetic susceptibility of the chemical intermediates which form during turnover of the water oxidizing site in photosynthetic membranes, 2) the application of EPR and ENDOR spectroscopies for determining the protein amino acids which bind manganese in the active site of the enzymatic cluster, and 3) the substitution of manganese by other metal ions as a structural and mechanistic probe of the water oxidizing reaction in photosynthetic membranes.